Organic solar cells (OSCs) and organic light emitting diodes (OLEDs) have gained increasing attention owing to their superior advantages of low cost, light weight, and flexibility of a variety of optoelectronic applications. One of the critical aspects in fabricating highly efficient and stable optoelectronic devices is the design of the functional carrier transport layers between the organic active layer and the electrodes. Typically, effective hole transport layers (HTLs) in optoelectronic devices have to satisfy electrical and optical requirements of (1) high electrical conductivity, (2) good optical transparency with wide bandgap, (3) good electron blocking with efficient hole transport. Poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) with a work function (WF) of 5.10 eV has been widely used as HTL in optoelectronic devices, such as conventional organic optoelectronics. However, there are some issues of its electrical and physical inhomogeneity, such as the long-standing acidic nature of PSS regarding the poor stability and severe degradation of organic optoelectronic devices. Alternatively, stable transition metal oxides (TMOs) such as MoO3, WO3, V2O5 and NiOx, stand out as promising candidates for efficient HTLs. It is essential to develop a wide range of efficient and low-cost TMOs to serve as functional HTLs. Until now, low-temperature solution-processed approaches have been demonstrated for MoO3, WO3, and V2O5.
Differently, the valence band of NiOx is well aligned with the highest occupied molecular orbital (HOMO) levels or valence band of many typical organic and inorganic semiconductors, respectively, for hole transport which is distinct from other typical oxide based HTL materials such as MoO3, WO3 and V2O5. Besides, NiOx offers promising characteristics as an anode interlayer with wide bandgap semiconductor properties, good electron blocking and optical transparency. However, the fabrication of highly efficient NiOx HTLs by a low temperature solution process still remains a great challenge. NiOx is a cubic rock-salt structure with octahedral Ni2+ and O2− sites. Pure stoichiometric nickel oxide is an excellent insulator with conductivity of 10−13 S cm−1, while non-stoichiometric NiOx is a wide bandgap p-type semiconductor. Due to the positive charge compensation induced by thermodynamically favored cation vacancies, the non-stoichiometric NiOx shows a p-type conduction property.
The conventional ways of fabricating NiOx usually involve thermal annealing processes and oxygen-plasma treatment, which hinders the applications of NiOx in flexible optoelectronic devices (i.e. OLED/OSC).